This invention relates to a method and apparatus for judging the purity of a purified zinc sulphate solution used for the electrolytic production of zinc, and more particularly to a method and apparatus of the kind above described in which the content of hydrogen gas in a gas mixture obtained by the electrolysis of the purified zinc sulphate solution is measured for judging the purity of the purified electrolyte.
It is commonly known that electrowinning of zinc from an acidic aqueous zinc sulphate solution thereof can be easily done utilizing such properties of zinc that zinc is a base metal and the hydrogen overvoltage of zinc is very high in the electrolysis. Zinc is thus generally produced by a electrolytic process in which such an electrolyte is subjected to electrolysis for the purpose of electrowinning of zinc. Therefore, the electrolyte supplied to the electrolysis section of the electrolytic zinc plant must be sufficiently purified to such an extent that it is substantially free from undesirable metal ions or ions of impurity metals which are separated from the electrolyte together with the zinc cathode in the electrolytic section of the electrolysis and tend to reduce the hydrogen overvoltage at the cathodes. This is because, when a purified zinc sulphate solution containing such impurity metal ions in large amounts is supplied to the electrolysis section, various troubles including undesirable reductions in the current efficiency and quality of products are given decrease to and a great length of time is required to recover normal state sources of electrolysis by removing of these troubles.
An effective means for judging the purity of the purified electrolyte supplied to the electrolysis section of the electrolytic zinc plant is essentially necessary due to the fact that the current efficiency in the electrolysis section is greatly adversely affected by the presence of undesirable impurity metal ions as pointed out above. A purity judging means has been proposed thitherto in which individual impurities are analyzed for judging the purity of the purified zinc sulphate solution. However, this prior art means has been defective in that the operation of analysis is quite troublesome and time-consuming, and the purity judged merely on the basis of the individual analyzed values is not always accurate and reliable due to the fact that the adverse effect of the impurities on the purity is variable depending on the interaction of these impurities.
Another purity judging means has been proposed which comprises suitably adjusting the acidity of the purified zinc electrolyte, disposing an anode and a cathode in a vessel having the electrolyte, supplying current across the electrodes, and directly measuring the hydrogen overvoltage at the cathode. However, this latter prior art means has also been defective in that a complex and expensive apparatus is required for the measurement and the result of the measurement is not always fully reliable. Thus, this latter means has not yet been widely employed in this field.
The present inventors have made a series of experiments and studies in an effort to obtain a highly effective and reliable means for judging the purity of the purified zinc sulphate solution. As a result of such effort, the present inventors have discovered that the content of hydrogen gas in a gas mixture produced by the electrolysis of the purified zinc sulphate solution (electrolyte) under predetermined electrolytic conditions has a fixed relationship with the current efficiency during the electrolysis of the purified acid aqueous solution as the electrolyte, and the lower the hydrogen gas content, the purity is higher and the current efficiency is improved.